Speaker
Description
Dedicated EAST experiments and modeling have been performed to investigate the influence of deuterium (D) and helium (He) plasmas on divertor detachment and tungsten (W) impurity transport. As fusion reactor will be operated in the D-T plasma with He naturally existing as the product of D-T reaction, the influence of different ion species on divertor detachment remains unclear. Density ramp-up discharges on EAST reveal that the divertor detaches at a relatively higher line-averaged electron density in He plasma than D plasma. With the same line-averaged electron density, He plasma exhibits higher electron density in the core and lower electron density in the scrape-off layer (SOL) than D plasma, which indicates different characteristics for achieving detachment. Meanwhile, divertor erosion and W impurity transport have been compared between D and He discharges. Under high-recycling divertor conditions, He induces W erosion rates are nearly six times higher than by D because of the higher sputtering yields, whereas the core W concentration in He plasma is only twice higher, indicating a better divertor W screening. Under detached divertor conditions, W erosion rate (dominated by impurities) is comparable in He and D plasmas. However, the resulting core W concentration in He plasma is merely half of that observed in D plasma.
To interpret the experimental observations, SOLPS and DIVIMP simulations [1] have been performed. Simulation results demonstrate that due to the higher ionization potential of He and He+ than D, He can penetrate deeper into the core plasma, resulting in a higher density in the core and a lower density in the SOL. E×B drifts show greater impact on plasma distribution of He than D, and therefore the relative difference in outer divertor detachment onset between He and D plasmas is more significant with favorable Bt direction compared to the unfavorable Bt direction.By leveraging the newly developed kinetic impurity transport model in DIVIMP, W transport and screening have been analyzed. The results show that W stagnation point in He plasma is located farther from the divertor targets than in D plasma, which is beneficial for a better divertor W screening. Moreover, the kinetic thermal force on W (pointing upstream) is reduced under conditions of lower ion temperature and higher effective charge (Zeff). This reduction enhances tungsten screening under detached divertor conditions, particularly in He plasmas, where the relatively higher Zeff further diminishes the thermal force.
[1] Q. Li, et al., Nuclear Fusion. 2025, 65, 026069